Handover (HO) indicates that a mobile station (MS) moves from an air interface of one base station to an air interface of another base station. Hereinafter, a handover procedure of a general IEEE 802.16e system will be described.
In an IEEE 802.16e network, a serving base station (SBS) may broadcast neighboring base station information through a neighboring advertisement (MOB_NBR-ADV) message in order to inform a mobile station (MS) of information (topology) about a basic network configuration.
The MOB_NBR-ADV message includes system information of a serving base station and neighboring base stations, for example, preamble indexes, frequencies, HO optimization degrees, downlink channel descriptor (DCD)/uplink channel descriptor (UCD) information, etc.
DCD/UCD information includes information necessary to enable an MS to perform information exchange in downlink and uplink. For example, DCD/UCD information includes HO trigger information, information about a medium access control (MAC) version of a base station, media independent handover capability information, etc.
A general MOB_NBR-ADV message includes only information about neighboring base stations of an IEEE 802.16e type. Information about base stations of the type other than an IEEE 802.16e type may be broadcast to MSs through a service identity information advertisement (SII-ADV) message. Accordingly, an MS may request an SBS to transmit the SII-ADV message so as to acquire information about base stations of a heterogeneous network.
A procedure for performing handover by an MS, which acquires information about neighboring base stations using the above-described method, in an IEEE 802.16e network will be described in greater detail.
The general procedure of performing handover in an IEEE 802.16e network may include HO initiation and preparation, HO execution and HO completion.
An example of a basic handover procedure having the above configuration will be described with reference to FIG. 1.
FIG. 1 is a diagram showing an example of a handover procedure which may be performed in an IEEE 802.16e system.
Referring to FIG. 1, first, a mobile station (MS) may access a serving base station (SBS) so as to perform data exchange (S101).
The SBS may periodically broadcast information about neighboring base stations (BSs) located within a coverage area thereof to the MS through an MOB_NBR-ADV message (S102).
The MS may begin to scan candidate HO BSs using an HO trigger condition while communicating with the SBS. The MS may transmit a handover request (MSHO-REQ) message so as to request the SBS to perform a handover procedure if a handover condition exceeds, for example, a predetermined hysteresis margin value (S103).
The SBS which receives the handover request may inform candidate HO BSs included in the MSHO-REQ message of the handover request from the MS through an HO-REQ message (S104).
The candidate HO BSs may take precautions for the MS which requests handover and transmit information associated with handover to the SBS through an HO-RSP message (S105).
The SBS may transmit information associated with handover acquired from the candidate HO BSs through the HO-RSP message to the MS through a handover response (BSHO-RSP) message. The BSHO-RSP message may include information for performing handover, such as an action time for performing handover, a handover identifier HO-ID and a dedicated HO CDMA ranging code (S106).
The MS may determine one target BS among the candidate HO BSs based on information included in the BSHO-RSP message received from the SBS. Then, the MS may attempt to perform initial ranging for transmitting a ranging request (RNG-REQ) message to the determined target BS using a CDMA code (S107).
The target BS which receives the CDMA code may transmit information as to whether or not ranging succeeds and physical correction values to the MS through a ranging response (RNG-RSP) message (S108).
If correction is not necessary, the MS may transmit a ranging request (RNG-REQ) message for authentication to the target BS (S109).
The target BS which receives the ranging request message of the MS may provide system information available to the BS, such as a connection identifier (CID), to the MS through a ranging response message (S110).
If the target BS successfully completes authentication of the MS and sends all update information, the target BS may inform the previous SBS of the MS of information indicating whether or not handover succeeds through a handover completion (HO-CMPT) message (S111).
Thereafter, the MS may exchange information with the target BS which performs handover (S112).
The above-described handover process is performed between the MS and the BS which follow the IEEE 802.16e standard (WirelessMAN-OFDMA Reference System). Hereinafter, in the present specification, for convenience, a system to which a general technology including the IEEE 802.16e standard is applied is referred to as a “legacy system” or a “first type system”. An MS to which the legacy technology is applied is referred to as an “YMS (Yardstick MS)”, a “legacy MS” or a “first type MS”, and a BS to which the legacy technology is applied is referred to as an “YBS (Yardstick BS)”, a legacy BS″ or a “first type BS”.
A system to which an advanced technology including the IEEE 802.16m standard (WirelessMAN-OFDMA Advanced System) is applied is referred to as a “new system” or a “second type system”. An MS to which the advanced technology is applied is referred to as an “advanced MS (AMS)”, a “new MS”, or a “second type MS”, and a BS to which the advanced technology is applied is referred to as an “advanced BS (ABS)”, a “new BS” or a “first type BS”.
It is assumed that an AMS accesses a YBS so as to receive a service from the YBS and an ABS (WirelessMAN-OFDMA Reference System/WirelessMAN-OFDMA Advanced co-existing system) supporting both an AMS and a YMS exists adjacent to the YBS.
The YBS has only a legacy zone (LZone) having a physical channel frame structure applied to a legacy system and the legacy zone refers to a zone supporting a legacy MS (YBS). It is assumed that an ABS has only a new MS support zone (MZone: 16M zone) having a physical channel frame structure applied to a new system if only an AMS is supported (WirelessMAN-OFDMA advanced system only). An ABS (WirelessMAN-OFDMA Reference System/WirelessMAN-OFDMA Advanced co-existing System legacy supportive) which supports both an AMS and an YMS has both a legacy zone (LZone) and a new MS support zone (MZone), which are divided in time units, for example, is divided using time division duplex (TDD) in frame units or subframe units, in uplink and downlink.
It is assumed that the AMS may receive services from both the ABS and the YBS. That is, it is assumed that the AMS may receive a service through any one of the new MS support zone and the legacy zone and may perform both a handover procedure defined in the legacy system and a handover procedure defined in the new system.
If the AMS performs handover to the ABS in a state of receiving a service from the YBS, the AMS may perform two handover procedures. If a target ABS supports a legacy zone (LZone), the ABS performs handover to an LZone of the target ABS through an IEEE 802.16e handover procedure and then completes the handover procedure through zone switching to MZone. If the target ABS does not support LZone, direct handover to an MZone may be performed.
Next, a capability negotiation method in a general IEEE 802.16e system will be described.
In an IEEE 802.16e network, capability negotiation between an MS and a BS is performed after initial ranging upon initial network entry. Capability negotiation between the MS and the BS is performed during initialization of the MS and is provided through subscriber station basic capability request/response (SBC-REQ/RSP) messages. An SBC-REQ message is a medium access control (MAC) message transmitted by the MS for basic capability negotiation with the BS, and an SBC-REQ message includes information about such capabilities, such as a modulation scheme and an encoding scheme supported by the MS.